find the standard deviation of all nodes in a linked list in php

main.php
class Node {
    public $value;
    public $next;
    
    public function __construct($value, $next=null) {
        $this->value = $value;
        $this->next = $next;
    }
}

class LinkedList {
    public $head;
        
    public function __construct() {
        $this->head = null;
    }
    
    // Function to calculate the standard deviation
    public function standardDeviation() {
        // Calculate the mean
        $count = 0;
        $mean = 0;
        $current = $this->head;
        while ($current != null) {
            $count++;
            $mean += $current->value;
            $current = $current->next;
        }
        $mean /= $count;
        
        // Calculate the variance
        $sum_squared_diff = 0;
        $current = $this->head;
        while ($current != null) {
            $sum_squared_diff += pow($current->value - $mean, 2);
            $current = $current->next;
        }
        $variance = $sum_squared_diff / $count;
        
        // Calculate the standard deviation
        $standard_deviation = sqrt($variance);
        
        return $standard_deviation;
    }
}

// Test
$list = new LinkedList();
$list->head = new Node(1);
$list->head->next = new Node(2);
$list->head->next->next = new Node(3);

echo $list->standardDeviation(); // Output: 0.81649658092773
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